This invention relates to signal processing and, more particularly, to methods of and apparatus for identifying training sequence information in a digital audio broadcasting signal.
There has been increasing interest in the possibility of broadcasting digitally encoded audio signals to provide improved audio fidelity. Several approaches have been suggested. One such approach, set forth in U.S. Pat. No. 5,588,022, which is hereby incorporated by reference, teaches a method for simultaneously broadcasting analog and digital signals in a standard AM broadcasting channel. An amplitude modulated radio frequency signal having a first frequency spectrum is broadcast. The amplitude modulated radio frequency signal includes a first carrier modulated by an analog program signal. Simultaneously, a plurality of digitally modulated carrier signals are broadcast within a bandwidth which encompasses the first frequency spectrum. Each of the digitally modulated carrier signals is modulated by a portion of a digital program signal. A first group of the digitally modulated carrier signals lies within the first frequency spectrum and is modulated in quadrature with the first carrier signal. Second and third groups of the digitally modulated carrier signals lie outside of the first frequency spectrum and are modulated both in-phase and in-quadrature with the first carrier signal.
The waveform in the AM compatible digital audio broadcasting system described in U.S. Pat. No. 5,588,022, was formulated to provide sufficient data throughput for the digital signal while avoiding crosstalk into the analog AM channel. Multiple carriers are employed by means of orthogonal frequency division multiplexing (OFDM) to bear the communicated information.
Digital audio broadcasting systems can transmit the digital information using various audio encoding and forward error correction rates to allow a broadcaster to trade-off audio quality for coverage area and resistance to channel impairments. The signals are subject to rapid changes in magnitude and phase. Training sequences can be used in systems such as AM In-Band On-Channel Hybrid digital audio broadcasting systems to allow a receiver to rapidly adapt to changes in the magnitude and phase of the received signal. In such systems there is a need for the receiver to identify when a training baud is received so that it can be properly processed. Such identification must be achieved even in the presence of high level interferers. This invention provides a novel method of identifying the reception of a training baud which is particularly applicable to digital audio broadcasting systems.
The present invention provides a method for identifying training information in a digital audio broadcasting signal. The method includes the steps of receiving a plurality of carrier signals modulated by a plurality of data baud, wherein the data baud includes normal baud and training baud. The data baud received on a plurality of carrier signals are compared with predetermined data signals to produce a plurality of difference signals. The difference signals are used to produce a plurality of distance signals, which are combined to produce a plurality of group distance signals. One of the group distance signals is selected and used to determine if the data baud corresponding to the selected group distance signal is a training baud or a normal baud. This determination is performed by storing successive selected group distance signals until at least one training baud has been received, and using one of the stored group distance signals to determine normal/training synchronization. The group distance signals correspond to data baud on predetermined groups of the carriers.
The invention also encompasses radio frequency receivers, which utilize the above method.